An annealing apparatus used for fabrication of semiconductors, flat panel displays, and solar cells is an apparatus which is in charge of an essential heat treatment step to perform processes such as crystallization, phase change, etc. on a thin film deposited on a substrate, such as a silicon wafer or glass.
For example, in a liquid crystal display and a thin film crystalline silicon solar cell, a silicon crystallization apparatus for crystallizing amorphous silicon deposited on a glass substrate into polysilicon is a typical annealing apparatus. Typically, a minimum temperature of 550 to 600° C. is required for crystallization of amorphous silicon.
That is, in general, an annealing apparatus is a heat treatment apparatus for applying heat to a thin film formed on a substrate, whereby heat is applied to the substrate as well as to the thin film.
However, a problem of this typical heat treatment method is that warpage of the substrate is caused by differences in thermal expansion coefficient between the substrate and the thin film in a cooling step after the heat treatment process. Particularly, in the case of using a glass substrate having a low melting point, such as flat plane displays or solar cells, warpage of the substrate is more problematic. This warpage of the substrate directly affects the productivity of flat plane displays or solar cells.
FIG. 1 is a view showing the problem of a conventional heat treatment method, wherein FIG. 1(a) is a view showing the state of a substrate before heat treatment, and FIG. 1(b) is a view showing the state of a substrate deformed by heat treatment.
As shown in FIG. 1(a), a crystallization heat treatment process is carried out under a condition where, for example, an amorphous silicon thin film 12 is formed on a glass substrate 10. When the crystallization heat treatment process is carried out under the condition of FIG. 1(a), this causes a problem, as shown in FIG. 1(b), that the substrate is deformed while being bent toward a portion having a low thermal expansion coefficient in a cooling step after the heat treatment process because the thermal expansion coefficients of the substrate 10 and the thin film 12 are different from each other.